1. Field of the Invention
The present invention relates to methods for the production of golf balls. More particularly, the present invention relates to methods for the production of a golf ball having a core and a cover comprising a thermoplastic resin composition.
2. Description of the Related Art
General golf balls have a core and a cover which covers the surface of this core. The cover usually comprises a thermoplastic resin composition. Injection molding or compression molding has been employed for formation of the cover.
Upon the injection molding, a resin composition is melted by heating in an injection molding machine. The melted resin composition passes a screw of the injection molding machine, and is injected toward a mold. The resin composition passes through a runner and a gate of the mold, and flows into a cavity at high speed. Thus entered resin composition is solidified to give a cover. For executing smooth inflow, the resin composition must have a low viscosity. Low viscosity can be achieved by elevating the temperature of the resin composition.
In recent years, covers of golf balls tend to be thin. Thin covers are accompanied by difficulty in formation. Various proposals in which improvement of formability in injection molding is intended have been made. U.S. Pat. No. 5,824,258 discloses a method of the production in which a mold having 10 or more gates is used.
In compression molding, the core is covered by two pieces of preforming material which comprises a thermoplastic resin composition and has a half shell shape. This core and the preforming material are placed into a mold. By clamping the mold, the preforming material is compressed. Concomitantly, the preforming material is heated. The compression and heating results in flow of the resin composition to form a cover. Also in formation of the preforming material, compression molding or injection molding has been employed.
Covers obtained by injection molding are inferior in abrasion resistance and scuff resistance. Upon hitting of the golf ball with a short iron, the cover tends to get scratched. In particular, this tendency is marked in covers having a thickness of less than 1.0 mm. Covers obtained by compression molding are more excellent in abrasion resistance and scuff resistance compared to the covers obtained by injection molding, although the level thereof is not satisfactory. An object of the present invention is to provide golf balls that are excellent in abrasion resistance and scuff resistance.